Analysis of a liquid composition to determine the presence and concentration of a particular chemical species is well-known. There are various techniques which have previously been used for this purpose, e.g., colorimetric analysis, titration to a particular pH, voltage (potentiometric titration), conductivity (amperometric titration), etc.
One or more of such techniques may be useful in analysis of a liquid composition containing an unknown concentration of a particular chemical species which is reactive with a reagent to produce a product which is different in at least one measurable physical property from (a) the particular chemical species in question, and (b) the reagent being used. For example, the resulting product may differ in terms of color, electrical potential, pH, conductivity, etc. By observing the occurrence of the reaction product it is possible to determine the amount of the particular chemical species which was present in the original sample.
The manner in which these processes have been carried out previously involves the addition of a single known amount of reagent to a solution containing a given amount, or concentration, of the chemical species to be determined, and then observing for the occurrence of the reaction product. For example, the occurrence of the reaction product may be noted by a color change, or pH change, conductivity change, etc.
An alternative method utilizes the incremental addition of reagent to a reaction vessel containing the chemical species in a manner such that the chemical species is consumed and a sudden property change is observed due to the complete absence of the free chemical species.
This incremental addition method is well known as titration. Known amounts of reagent are added to the solution to consume the chemical species in question. When the particular chemical species is completely consumed (i.e. no free chemical species remaining), a sudden and complete change in some physical property is noted. Then the concentration of the chemical species is calculated as a function of the amount of reagent added to produce the change (or endpoint) rather than as a function of the magnitude of the change after the addition of a uniform amount of reagent.
The property which changes to signify the endpoint may be color (colorimetric titration), pH, voltage (potentiometric titration), conductivity (amperometric titration), etc.
The addition of reagent to the solution in the previously known procedures referred to above may be performed manually, if desired. More recently there have been proposed devices which automatically inject reagent into a reaction vessel in which the solution to be tested is contained. Such devices are commercially available and involve a syringe filled with the desired reagent to be used. A piston in the syringe is caused to move by means of a controlled stepper motor to force reagent out of the syringe.
The controlled stepper motor can rotate in precise increments, and such rotation is translated (via a screw mechanism) to a linear motion which drives a piston in a syringe. The motor can actuate in both directions and thus can fill as well as dispense, although a diverting mechanism must be used so that the syringe can fill from one vessel and dispense into a separate (reaction) vessel. The piston in the syringe requires seals at the piston head. Such seals can develop leaks with passage of time (due to cold-flowing of the seal to release tension) and with continued use (due to scarring of the seal or the barrel wall by particulates).
There has not heretofore been provided systems or apparatus for automatic analysis of solutions having the advantages of the present invention.